Personally, I think AirTran should either assign all seats or keep it open until the 24 hr mark like they used to. Personally, I would prefer the latter. The 24 hour check in to reserve seats worked just fine for me. This pay extra policy just sets up for issues.
That said, it is the policy, and if you really are worried about it, you should pay and have the seats reserved. At the very least, check your flight online periodically to see if what the seat availability is. If things look open (i.e., many open seats left) you can maybe feel comfortable that you can wait for the 24-hr check in to choose your seats. If seats are getting tight, you may want to pay to reserve them for peace of mind. I have a flight booked down to MCO from White Plains in late April and did not initially reserve seats. I have checked every week or so to see how many open seats were left, and recently decided that the flight down was getting too tight and went ahead and reserved them. The flight back remains quite open, however, and if it stays that way, I will just do the 24-hr check in on the way back.
As for those digging their heels in about not moving, I agree that there is no excuse for people who know the policy and choose to cheap out and take their chances. But I suspect many people (although obviously not the OP or others on this board), especially infrequent fliers or those new to AirTran, don't really understand this (unique) policy and get caught with their pants down. I therefore would consider moving if the situation was right. Not to mention, I would have a hard time punishing a terrified 3-year old for the ignorance or arrogance of their parents. Of course, a 6-7 year old is probably old enough to put on the big girl/boy pants and be separated from their parents for a while.